Heated rolls are used in the laminating process where a thin protective thermoplastic film is applied to an article. The article to be laminated may include such things as paper documents and packaging for food stuffs.
The laminating process typically involves passing the article and the thermoplastic film through adjacent heated nip rolls which fuse the film directly to the article by the application of heat and pressure. Once the article has passed through the heated nip rolls, the thin protective thermoplastic film is essentially adhered to the article.
Most existing laminating machines use heated nip rolls that include a cylindrical core that has a silicone rubber cover. The heated nip rolls are typically heated internally through the use of heated liquids. The liquid may be heated internally within the roll through the use of heating elements. More commonly, the liquid is heated externally in a special heater.
Although heated nip rolls using heated liquid are generally effective at maintaining nip rolls at a constant temperature necessary for laminating, there are significant disadvantages to this technology.
A first disadvantage of using a heated liquid to heat laminating machine nip rolls is the length of time required to heat the liquid and thus the roll to operating temperature. This delay forces machines to be turned on well in advance of using the machine. Once the machine is at operating temperature the machine can not typically be shut down as the heat up time required would cause extensive delays.
Laminating roll heating systems of this type using a heated liquid as the means to heat the rolls are often used in work environments where the laminating machine is only used intermittently throughout the work day. A typical work environment where laminators are used is a copy or print shop. The laminating machine could be used for large production runs such as menus or for small jobs such as laminating a single document. Keeping a laminating machine at operating temperature all day to be able to perform small intermittent jobs adds a significant expense as the power requirements of keeping laminating machines of this type at operating temperature are quite high.
Another disadvantage of laminating machines that use a heated liquid to heat the nip rolls is the unsafe nature of hot liquids. Should a pressurized hose or cylinder rupture, the hot oil or glycol used in the heating the rolls would be likely to uncontrollably spray out of the system and inflict severe injuries and property damage.
Heated laminating rolls have previously been developed that use resistance wires to heat the laminating rolls. The resistance wires on these laminating rolls have been disposed on the core of the laminating roll underneath a covering of silicone rubber.
An advantage of using resistance heater wires is that resistance heater wires are much quicker at bringing a laminator roll to operating temperature than heated liquid systems. Because of this the laminating machine does not need to be maintained at operating temperature at all times. This feature saves considerable expense over heated liquid systems due to the decreased power usage. Laminating machines maintained at operating temperature often contribute a large amount of heat to work environments. This extra heat increases the cooling requirements of air conditioning equipment and the electrical power consumption expense associated therewith.
Laminating machines using resistance heater wires have fewer space requirements compared to a heated liquid system that has an external liquid heater. This feature also makes the laminating machines using resistance heater wires more easily accommodated into a work space.
Laminating machines using resistance heater wires to heat the laminating rolls also use a rolls that are considerably lighter in weight than the rolls used in heated liquid type machines. The extra weight of rolls using heated liquid requires that the entire laminating machine is also built of a heavy construction to properly support the heavy rolls. This extra weight also requires heavier drive mechanisms and heavier duty drive motors. Because of the overbuilt nature of heated liquid laminating machines the machines are less portable than resistance heater wire systems and are much more costly to manufacture.
In addition to the decreased materials required, laminating machines using resistance heater wires generally are also much simpler to manufacture compared to machines using heated liquid as a means to heat the rolls. This simplicity presents significant cost savings to the manufacturer. There are also corresponding savings to the operators of the laminating machines using resistance heater wires that are derived from a substantial decrease in the maintenance requirements of these laminating machines.
There are also safety benefits of using resistance heater wires to heat laminating rolls. There are no hot pressurized liquids that could inflict severe injuries or property damage if a component of the system ruptures. There is a significant decrease in safety inspections for resistance heater wire laminating machines as the machines do not use pressurized heated liquids.
Although there are significant benefits to using resistance heater wires as a means to heat rolls in laminating machines, there have also been two significant disadvantages that were not overcome by the prior art.
A first disadvantage of laminating rolls using resistance heater wires is due to the uneven application of heat to the rolls by the resistance wire. Attempts at conducting heat evenly to the rolls in previous systems have comprised the impregnation of the silicone rubber roll covering with copper particles or the use of metallic compounds electro-sprayed over the resistance wires. Impregnating silicone rubber with copper particles provided an increase in the heat conductivity of the silicone rubber and a more even heat distribution on the rolls. In this way the disadvantage of uneven heat distribution was overcome with this design. However, this system was unsuccessful because mixing copper particles into silicone greatly restricts the methods of applying silicone to the core of a laminating roll during the manufacturing process. Additionally, the silicone is difficult to machine to a necessary surface finish due to the inclusion of copper particles which cause excessive tearing and pitting at the outer machined surface of the silicone. As a very smooth surface finish is required for high quality laminating the process proved to be undesirable.
Electro-spraying laminating rolls with copper to increase the heat conductivity has also proven to be unsuccessful as the additional manufacturing costs are prohibitive. Furthermore, metallic coatings produced by electro-spraying are quite brittle and do not hold up well to the rigors of pressure and temperature shock that effect laminating rolls.
An additional problem with the prior art laminating rolls using electrical resistance heater wires has been the safety risk associated with working around live electrical wires that are covered only with the silicone rubber covering on the roll. The risk with this system is due to the sharp tools often used in conjunction with laminating machines. These tools may penetrate into the silicone where the tool may contact a live resistance wire causing injury to the user.
For the foregoing reasons there is a need for a heated laminating roll that utilizes the substantial benefits provided by electrical resistance heater wires but additionally provides a means to conduct the heat uniformly throughout the roll. There is also a need for a heated laminating roll that provides a safety mechanism to ensure that a user of the laminating machine cannot come in contact with a live electrical resistance wire.